O-ring is a sealing element with a circular cross-section shape, which is widely used in various mechanical equipment. It has many advantages, including simple structure, compact installation part, light weight, self-sealing effect, good sealing performance, small friction resistance and so on. The O-ring can be applied to seal all liquids and gases in practical use, and can be used for a variety of static or dynamic applications. It is made of elastomer material, vulcanized in a mold, with a round appearance and a ring cross-section. O-rings are available in a variety of sizes, with a cross-section diameter of about 0.35~40 mm and an inner diameter of 5,000 mm and above.
Working principle
The O-ring is a bi-directional sealing element, and its initial compression determines the initial sealing capacity. The force generated by the system pressure acting on the O-ring is the total sealing force, and the sealing force increases with the increase of the system pressure. Under pressure, the shape of the O-ring is similar to that of a liquid with high surface tension, and the pressure is transmitted equally in all directions.
Choosing the right O-ring material is very important to ensure the sealing effect and extend the service life.
ACM (polyacrylate rubber) : has excellent ozone resistance, weathering resistance and heat resistance to air, but its physical strength is medium, poor elasticity, and low temperature resistance is relatively weak. The operating temperature range is -20°C to +150°C, which can reach +175°C in a short time, mainly used in the automotive industry.
CR (chloroprene rubber) : has good ozone resistance, climate resistance, chemical resistance and aging resistance, but also has good flame retardant, mechanical properties and cold flexure. The operating temperature range is -35°C to +90°C (+120°C for a short time), and the operating temperature of special types of chloroprene rubber can be as low as -55°C.
EPDM (EPDM) : has good heat resistance, ozone resistance and aging resistance, but also has good elasticity, good low temperature performance and insulation properties. The operating temperature range is -45°C to +150°C (up to +175°C in a short time), and the operating temperature range of the vulcanized EPDM is reduced to -45°C to +130°C (up to +150°C in a short time).
FFKM (perfluorinated rubber) : Similar to polytetrafluoroethylene materials, has a wide range of chemical and heat resistance, and exhibits very low expansion for all media. The operating temperature is -25°C to +240°C, and special types of perfluorinated rubber can operate up to +325°C.
FKM (fluorine rubber) : Depending on its structure and fluorine content, its chemical resistance and cold deflection are also different. It has flame retardant, very low permeability and excellent ozone resistance, weathering resistance and aging resistance. The operating temperature range is -20°C to +200°C (up to +230°C for a short period of time), and properly formulated fluororubbers can operate at low temperatures of -35°C.
FVMQ (fluorosilicone rubber) : has good heat resistance, very good elasticity at low temperatures, also has good electrical properties and excellent weather resistance, ozone resistance and ultraviolet (UV) resistance. The application temperature ranges from -50°C to +175°C (up to +200°C in the short term).
HNBR (Hydrogenated nitrile butadiene rubber) : has good mechanical properties, when in contact with mineral oil and lubricating grease, the operating temperature range is -30°C to +140°C (can reach +160°C in a short time), special types of hydrogenated nitrile butadiene rubber operating temperature can be as low as -40°C. IIR (butyl rubber) : has very low air and water permeability, but also shows very good resistance to organic and inorganic chemical media, ozone, climate and aging resistance. Operating temperatures range from -40 °C to +110°C (but can reach +120°C for a short time).
NBR (Nitrile butadiene rubber) : has good mechanical properties, the operating temperature range is -30°C to +100°C (can exceed +120°C in a short time), after proper formulation of nitrile rubber can work at a low temperature of -60°C.
Polyurethane (polyurethane) : It has outstanding tensile strength, low permanent compressive deformation, and good resistance to oxygen and ozone. Application temperatures can range from below -50°C to +110°C, and can be applied to higher temperatures in the short term.
VMQ (silicone rubber) : has excellent heat resistance, cold flexure, insulation properties and excellent oxidation resistance, ozone resistance and ultraviolet (UV) resistance. The operating temperature range is -50°C to +175°C (+230°C in a short time), and the operating temperature of special types of silicone rubber can be as low as -90°C.
Suggestions for installation methods and sealing groove design
During the design phase, the installation of the O-ring should be considered to avoid the O-ring passing through the sharp edge or the inner hole. When the sliding distance is long, the seal seat should be designed as a groove as possible, or the O-ring should be properly adjusted to ensure that it only needs to slide a short distance during installation to reduce the risk of distortion of the O-ring.
Radial installation (static and dynamic motion) : internal thorough seal (piston rod seal) : When selecting an O-ring of appropriate specification, ensure that its outer diameter (d1+2*d2) is at least equal to or greater than the groove outer diameter d6. External thorough seal (piston seal) : When selecting an O-ring of appropriate specifications, ensure that its inner diameter d1 is at least equal to or less than the groove inner diameter d3.
Axial installation (static motion) : In the axial static installation, the direction of the pressure should be considered when selecting the appropriate specification of the O-ring. When there is internal pressure, the outer diameter of the O-ring should be equal to or greater than the outer groove diameter d7; When external pressure is present, the inner diameter of the O-ring should be less than the diameter of the inner groove d8.
Initial compression: The initial compression of the cross section of the O-ring in the groove is critical to ensure its role as a primary or secondary sealing element. Depending on the application, the recommended initial compression amount is dynamic application: 6~20%; Static application: 15~30%.
Compression force: The compression force of the O-ring is variable, mainly depending on the initial compression amount, material type, material hardness, O-ring inner diameter and its cross section and other factors.
Groove filling: The recommended groove filling ratio shall not exceed 85% of the thermal expansion or volume expansion due to fluid contact and tolerance effects.
Surface finish of O-ring grooves: Rectangular grooves are preferred for all new designs, and this structure allows for up to 5° bevels on both sides of the grooves. If a retainer is used, the vertical side must be used. Trapezoidal grooves are used only in special cases, such as in top-mounted cases, in order to hold the O-ring in place.
O-ring seal design is a sealing technology widely used in various industrial fields. It has the advantages of simple structure, convenient installation and reliable sealing performance. When designing O-ring seals, it is necessary to consider a variety of factors, such as sealing medium, working pressure, working temperature, sealing gap, etc. Through reasonable selection of O-ring material, size and compression, good sealing effect can be achieved, and the reliability and service life of the equipment can be improved.
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